The invention starts from a process for producing granules with a narrow particle size distribution in which the liquid starting product is sprayed into a fluidized bed and the finished granular particles are discharged from the fluidized bed with classification on the zigzag classifying principle. The invention also relates to a device for carrying out the process.
In modern processes for fluidized bed spray granulation the granules are produced in one process stage from the liquid starting product, there being a classifying discharge from the fluidized bed. It is therefore possible to dispense with external classifying and grinding of the oversize particles. This process requires a classifying discharge which releases the particles which have achieved the required particle size from the fluidized bed. A process of this type is, for example, described in EP 163,836.
A granulator of circular cross-section is known from DE 2,555,917 in which the classifying discharge device is positioned in the centre of the fluidizing base (inlet plate). The inlet plate is formed from a perforated plate which has increasing density of perforation from the inside to the outside. This has the effect that the particles rise at the container wall and fall in the centre.
In another fluidized bed granulator described in DE 1,667,217 a so-called spouted bed is produced for active blending of the bed contents. This produces a forced particle circulation which is exactly the opposite of the one described above. The circular conical constriction of the granulator at the base is a determining factor for this particle circulation. At the lower end of the conical constriction is attached a tube through which the fluidizing air passes into the granulator. A jet channel is produced at the axis of the fluidized bed in which the particles are dragged upwards. Above the bed the particles fall out of the jet and back into the bed. There is thus a particle circulation in which the conical base promotes the transfer of the particles into the jet channel. The particles which have achieved the required particle size fall down within the air supply tube and are transferred out of the granulator. This type of granulator has proved to be effective for relatively small cross-sections; however, problems and difficulties appear with the large throughputs which can only be achieved with correspondingly large cross-sections.
The particle circulation described previously in which the particles fall at the granulator axis is preferably combined with injection of the liquid product from above, while a particle circulation which rises at the granulator axis is required for injection from below.
It is known that in general there are difficulties in scaling-up for gas/solid/oil beds (see, for example, Aufbereitungstechnik No. 12 (1974) pages 670 to 677). The similitude theory which is often employed at other times for process engineering plants fails in this case. It has been found that in fairly large fluidized bed granulators with classifying discharge the particles travel through the fluidized bed on longer paths and therefore no longer participate so frequently in the classifying process. This increases the risk of forming oversized particles. In unfavourable situations coarse particles can also fall into the fluidized bed here and concentrate in the vicinity of the inlet plate. In extreme cases the fluidization can "go to sleep" here. The coarse particles then settle on the inlet plate and increase still further in size through the addition of other particles. The resulting large aggregates (lumps) come into contact with the hot fluidizing air and suffer thermal deterioration. They also disrupt the flow through the bed. Under some circumstances this disruption can even spread to total collapse of the fluidized bed. The size of the particle contained in the bed must therefore be checked at short intervals if trouble-free operation is to be maintained.
This is where the invention come into effect.